What is Quantum Computation?

Whereas classical information is based on bits taking only two possible values, quantum information relies on qubits: a qubit generalizes the concept of a bit since it may not only attain two values |0> and |1>, say, but also “superpositions” of them, i.e., vector sums like |0> + |1> or |0> - |1>. This is a radically new logical property, totally unknown in classical information processing. It enables new kinds of logical gates and algorithms, working on quantum registers, ordered collections of several qubits.

Although the computation of a whole quantum circuit or algorithm can be highly nonclassical, both input and output must be classical information, encoded by bits. This is obvious because we want algorithms to solve questions that we have, and we have them “classically,” and we need information in a classical form since this only answers our question.² To receive the output, the quantum register, or parts of it, has to be measured. Measurement is the only way to receive classical information from quantum systems.

To summarize, a quantum computer has bits as input, it works with qubits, and gives bits as output.